At what age is it appropriate to start leash pressure training? 

The age of the dog is really dependent on 1) the ultimate goals for training and 2) the individual dog and how sensitive or motivated they are.

I'm going to start leash pressure earlier with a pushy, insensitive pet dog than I would with a reserved, less motivated dog. The age is typically after teething but not always.

For a dog that I plan on competing in a sport with, I might wait longer and get my engagement and play relationship solid before doing leash pressure as it can be mildly stressful for some dogs.

I hope that makes sense.